The significant polarization components of a medium produced by contact with an electric field are first order polarization (linear polarization), second order polarization (first nonlinear polarization), and third order polarization (second nonlinear polarization). On a molecular level this can be expressed by Equation 1: EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3. . . (1)
where
P is the total induced polarization, PA1 E is the local electric field created by electromagnetic radiation, and PA1 .alpha., .beta., and .gamma. are the first, second, and third order polarizabilities, each of which is a function of molecular properties. PA1 P is the total induced polarization, PA1 E is the local electric field created by electromagnetic radiation, and PA1 .chi..sup.(1), .chi..sup.(2), and .chi..sup.(3) are the first, second, and third order polarization susceptibilities of the electromagnetic wave transmission medium.
.beta. and .gamma. are also referred to as first and second hyperpolarizabilities, respectively. The molecular level terms of Equation 1 are first order or linear polarization .alpha.E, second order or first nonlinear polarization .beta.E.sup.2, and third order or second nonlinear polarization .gamma.E.sup.3.
On a macromolecular level corresponding relationships can be expressed by Equation 2: EQU P=.chi..sup.(1) E+.chi..sup.(2) E.sup.2 +.chi..sup.(3) E.sup.3. . . (2)
where
.chi..sup.(2) and .chi..sup.(3) are also referred to as the first and second nonlinear polarization susceptibilities, respectively, of the transmission medium. The macromolecular level terms of Equation 2 are first order or linear polarization .chi..sup.(1) E, second order or first nonlinear polarization .chi..sup.(2) E.sup.2, and third order or second nonlinear polarization .chi..sup.3 E.sup.3.
To achieve on a macromolecular level second order polarization (.chi..sup.(2) E.sup.2) of any significant magnitude, it is essential that the transmission medium exhibit second order (first nonlinear) polarization susceptibilities, .chi..sup.(2), greater than 10.sup.-9 electrostatic units (esu). To realize such values of .chi..sup.(2) it is necessary that the first hyperpolarizability .beta. be greater than 10.sup.-30 esu.
What is required in a practical sense to build optical articles containing an organic transmission medium that exhibits a high (i.e., greater than 10.sup.-9 esu) second order polarization susceptibility are a combination of features. First, organic molecular dipoles are required that exhibit high (&gt;10.sup.-30 esu) hyperpolarizability. A variety of efficient organic molecular dipoles are known. They include an electron donor moiety linked to an electron acceptor moiety through a conjugated .pi. bonding system to permit oscillation of the molecular dipole between a lower polarity ground state and a higher polarity excited state. Although improvements in organic molecular dipoles are continuing, existing organic molecular dipole chromophores permit selection from a variety of adequate structures. Second, to translate the high hyperpolarizabilities of the molecular dipoles into a high (&gt;10.sup.-9 esu) second order polarization susceptibility transmission medium, it is necessary to achieve both a high density of the organic molecular dipoles in the transmission medium and a high degree of polar alignment of the organic molecular dipoles within the medium. Many otherwise promising materials have fallen short of practical needs in satisfying these requirements. Third, the transmission medium must be transparent to the wavelength or wavelengths of electromagnetic radiation to be transmitted.
In an attempt to meet these varied requirements the art has paid particular attention to attempting to attach organic molecular dipoles as pendant groups to the backbone of linear vinyl polymers. Linear vinyl polymers offer a variety of advantages in that they are generally transparent and have good rheological properties, being readily coated, usually from solution. Further, linear vinyl polymers with glass transition temperatures above ambient are available, allowing polar alignment of pendant molecular dipoles at elevated temperatures and cooling to room temperature to reduce the freedom of the pendant molecular dipoles to revert to random orientations.
Not withstanding the promise of linear vinyl polymers with pendant organic molecular dipoles in producing high .chi..sup.(2) transmission media for optical articles, a number of difficulties have been encountered. First, attempts to form organic molecular dipole substitutions of linear vinyl polymer repeating units have often been limited to a fraction of theoretically available sites. This has limited the density of organic molecular dipole incorporation and has a direct adverse effect on the .chi..sup.(2) of the transmission medium. Another difficulty has been that the organic molecular dipole substituted linear vinyl polymers have often exhibited low glass transition temperatures, which have allowed the molecular dipoles freedom of mobility from their desired polar aligned orientation. Still another difficulty has been lack of the desired degree of transparency in the visible (400 to 700 nm) region of the spectrum.
The difficulties encountered by the art can be appreciated by considering the shortcomings of attempts to form high second order polarization susceptibility transmission media by attaching organic molecular dipoles to polystyrene, reported by
R-1 C. Ye, T. J. Marks, J. Yang and G. K. Wong, "Synthesis of Molecular Arrays with Nonlinear Optical Properties. Second Harmonic Generation by Covalently Functionalized Glassy Polymers", Macromolecules, Vol. 20, pp. 2322-2324, (1987) and
R-2 C. Ye, T. J. Marks, J. Yang and G. K. Wong, "Synthesis Approaches to Stable and Efficient Polymeric Frequency Doubling Materials. Second-Order Nonlinear Optical Properties of Poled, Chromophore-Functionalized Glassy Polymers", Nonlinear Optical Effects in Organic Polymers (J. Messier et a. eds.), pp. 173-183, Kluwar Academic Publishers (1989).
In R-1 polystyrene was provided with an iodomethyl functional group in the para ring position that interacted with a thallium oxide functional group attached to an organic molecular dipole or a pyridinium ring of an organic molecular dipole to produce a linear vinyl polymer having pendant organic molecular dipoles covalently bonded through methylene linkages. R-1 states, "Most experiments have been carried out with polymers having 4.5-12.5% (by elemental analysis and NMR) of the benzene rings functionalized." That is, very low levels of organic molecular inclusion were achieved. This is revealed in terms of low values of .chi..sup.(2) (reported as d.sub.33) only slightly exceeding 10.sup.-9 esu for the thallium prepared polymer at the highest poling potential and generally an order of magnitude lower for the pyridinium containing molecular dipole polymer. Thus, the pyridinium molecular dipole containing polymer exhibited too low a .chi..sup.(2) value to permit practical use. The thallium prepared polymer suffered from the obvious toxicity risks associated with thallium compounds. Further, the resulting organic molecular dipole substituted polystyrene produced by the thallium procedure was objectionably colored (purple).
R-2 reports the attachment of the chromophores N-(4-nitrophenyl-L-prolinol), referred to as NPP, and 4-(4-nitrophenylaza-N-ethyl-2-hydroxyethyl)aniline, referred to as Disperse Red or DR, to polystyrene using two different attachment schemes. In Scheme I the chromophore is provided with a toluene sulfonate functional group and the styrene with a 4 ring position hydroxy substituent that interact to allow attachment through an oxy linkage. Scheme II is similar to that of R-1 above providing the chromophore with a hydroxy functional group that reacts with a chloromethyl 4 ring position substituent of the polystyrene to produce a methylene linkage. R-2 states that using the Scheme I oxy linkage organic molecular dipole attachment densities can be increased to 60 percent of the polystyrene repeating units (still well below desirable high levels of substitution), but in Table I no functionalization of greater than 50 percent is reported, with the lowest Scheme II functionalization reported being only 4.5 percent. Even at the highest reported level of poling the .chi..sup.(2) values were did not exceed a modest 1.8.times.10.sup.-8 esu. A further problem is revealed in the reported poling temperatures of 80.degree. and 110.degree. C. These low poling temperatures are indicative of objectionably low T.sub.g polymers and lack of polar alignment stability at ambient temperatures. Finally, the films produced were objectionably colored. The films containing the NPP chromophore were yellow-orange while those containing the DR chromophore were purple.